An Orthogonal Frequency-Division Multiplexing (OFDM) method is one of digital multi-carrier transmission methods adopted in various digital communications compliant with a Japanese digital terrestrial broadcasting standard (Integrated Services Digital Broadcasting-Terrestrial, or ISDB-T), a European digital terrestrial broadcasting standard (Digital Video Broadcasting-Terrestrial, or DVB-T), the IEEE 802.11a standard, etc. With the OFDM method, a plurality of carriers are closely arrayed while remaining orthogonal to each other. Therefore, the OFDM method enables efficient use of frequency. Furthermore, as the OFDM method allows setting long symbol lengths, the OFDM method is robust against InterSymbol Interference (ISI) caused by a plurality of incoming waves.
A guard interval technique is commonly used in the OFDM method. The guard interval technique makes it possible to perform the Fast Fourier Transform (FFT) while avoiding interfering components associated with the ISI caused by multipath interference, by extracting a portion of the end of a useful symbol and inserting the extracted portion to the front of the useful symbol as a guard interval, so as to maintain periodicity in the useful symbol. Adopting the guard interval technique in the OFDM method renders the OFDM method significantly robust against multipath interference.
According to the aforementioned ISDB-T and DVB-T standards, a transmission signal is configured in a transmission format that is schematically shown in FIG. 41. Referring to FIG. 41, the horizontal axis and the vertical axis indicate the carrier (frequency) direction and the symbol (time) direction of an OFDM signal, respectively. According to the ISDB-T and DVB-T standards, a pilot signal, which is called a Scattered Pilot (SP) and referenced when performing equalization, is allocated to every 12th carrier in each symbol. Hereafter, such a pilot signal is referred to as a “scattered pilot signal” or an “SP signal”. In the carrier direction, each SP signal in one symbol is two carriers apart from the corresponding SP signal in another adjacent symbol. This way, in every 4th symbol, each SP signal is assigned the same carrier number as the corresponding SP signals in other symbols. With the SP signals allocated in the above-described manner, the transmission signal shown in FIG. 41 is transmitted. The amplitudes and phases of the SP signals are known to a receiver. Below, a carrier to which an SP signal is allocated is referred to as an “SP carrier”. Note, an SP signal allocation pattern (a pattern in which SP signals are allocated) pertaining to the Digital Video Broadcasting-Handheld (DVB-H) is the same as the SP signal allocation pattern pertaining to the ISDB-T and DVB-T standards.
Once having been transmitted, a transmission signal X is affected by various amplitude distortions and phase distortions in a channel due to multipath interference, fading, and the like. The transmission signal X is then received by a receiver as a reception signal Y. The transmission signal reflects off various objects while being transmitted. As a result, the transmission signal is transmitted via multiple paths and arrives at the final destination at different times. That is to say, in such a multipath environment, channel characteristics show delay spread. On the other hand, in a mobile environment, a transmission signal is affected by the Doppler shift, and a plurality of waves travelling in different directions overlap one another. That is to say, in such a mobile environment, channel characteristics show Doppler broadening. Given that the characteristics of amplitudes and phases by which the transmission signal X has been affected in the channel (channel characteristics) are expressed as H, the transmission signal X and a reception signal Y satisfy the relationship shown in the following (Equation 1).Y=HX  (Equation 1)
In view of the above, the receiver estimates the transmission signal X by (i) estimating the channel characteristics Hand (ii) correcting the amplitude distortions and phase distortions by which the reception signal Y has been affected in the channel, by multiplying the reception signal Y by the reverse characteristics of the channel characteristics H. In the ISDB-T and DVB-T standards, the channel characteristics H are estimated using the above-described SP signals. There are various receiving environments that the receiver could be in. Many of such receiving environments cause a long delay in signal reception. Examples of such receiving environments include a Single-Frequency Network (SFN) environment where a plurality of transmission stations transmit signals using the same frequency, and a receiving environment where when the receiver receives radio waves that have reflected off remotely-located reflectors. In addition, there has been an increasing demand for a wider variety of reception techniques that allow users to receive signals while travelling (e.g., in a running car). Accordingly, robustness against delay spread and Doppler broadening is desired in the field of channel characteristics estimation.
With reference to FIG. 42, the following describes general processing for correcting amplitude distortions and phase distortions in the ISDB-T and DVB-T standards. In a receiver 1000, an FFT unit 1001 separates a reception signal into a plurality of carriers by performing the Fast Fourier Transform (FFT) on the reception signal on a per-symbol basis. A channel characteristics estimation unit 1003 estimates channel characteristics using SP signals included in the signals output from the FFT unit 1001. An equalization unit 1002 corrects the amplitude distortions and phase distortions by which the signals output from the FFT unit 1001 have been affected, by multiplying the signals output from the FFT unit 1001 by the reverse characteristics of the estimated channel characteristics. In the above manner, the equalization unit 1002 estimates the transmission signal.
In general, there are following two basic methods of estimating channel characteristics using SP signals.
A description is now given of the first method of estimating channel characteristics with reference to FIGS. 43, 44A and 44B. FIG. 43 shows the structure of the channel characteristics estimation unit 1003. In the channel characteristics estimation unit 1003, an SP channel characteristics estimation subunit 1010 (i) extracts SP signals from the signals output from the FFT unit 1001, (ii) generates reference SP signals that are known to the receiver (i.e., SP signals whose amplitudes and phases are the same as those of the SP signals generated by the transmitter), (iii) divides each of the extracted SP signals by a corresponding one of the generated reference SP signals, and (iv) outputs, to a symbol direction interpolation subunit 1011, each result of the division as a value of channel characteristics at the SP carrier to which the corresponding SP signal is allocated. The symbol direction interpolation subunit 1011 calculates values of channel characteristics at carriers whose carrier numbers are “0” or a multiple of “3” (hereafter, “3nth carriers” with n being an integer equal to or larger than “0”), by interpolating, in the symbol (time) direction, the values of channel characteristics at the SP carriers, which have been output from the SP channel characteristics estimation subunit 1010 (see FIG. 44A). Thereafter, a carrier direction interpolation subunit 1012 calculates values of channel characteristics at all the carriers by interpolating, in the carrier (frequency) direction, the values of channel characteristics at 3nth carriers, which have been output from the symbol direction interpolation subunit 1011 (see FIG. 44B). As the first method estimates channel characteristics using SP signals each of which is allocated to every 4th symbol in the symbol direction, the first method is referred to as “four symbol estimation” below.
As opposed to the above first method, the second method does not interpolate the channel characteristics at the SP carriers in the symbol direction. Instead, the second method calculates channel characteristics at all the carriers by interpolating, only in the carrier direction, the channel characteristics at the SP carriers that are eleven carriers apart from one another in each symbol. As the second method estimates channel characteristics using SP signals included in every one of the symbols, the second method is referred to as “one symbol estimation” below.
A description is now given of the one symbol estimation with reference to FIGS. 45 and 46. FIG. 45 shows the structure of a channel characteristics estimation unit 1003a. In the channel characteristics estimation unit 1003a, an SP channel characteristics estimation subunit 1010 (i) calculates channel characteristics at the SP carriers in the above-described manner, and (ii) outputs, to a carrier direction interpolation subunit 1012a, values of the calculated channel characteristics at the SP carriers that are eleven carriers apart from one another in each symbol. The carrier direction interpolation subunit 1012a calculates values of channel characteristics at all the carriers by interpolating, in the carrier direction, the values of the channel characteristics at the SP carriers output from the SP channel characteristics estimation subunit 1010, the SP carriers being eleven carriers apart from one another in each symbol (see FIG. 46).
The aforementioned four symbol estimation and one symbol estimation have the following features. Note, given that an OFDM useful symbol length, an OFDM symbol length and a guard interval length are respectively expressed as Tu[s], Ts[s] and Tg[s], they satisfy the relationship shown in the following (Equation 2).Ts=Tu+Tg  (Equation 2)
In the case of four symbol estimation, the symbol direction interpolation is performed by using only the channel characteristics at the SP carriers that are three symbols apart from one another in the symbol direction. Hence, according to the sampling theorem, the symbol direction interpolation is performed using a filter whose passband is equal to or smaller than 1/(4 Ts) [Hz] as shown in FIG. 47A. For this reason, in the case of four symbol estimation, the symbol direction interpolation can be performed without being affected by aliasing as long as the Doppler broadening is equal to or smaller than 1/(4 Ts) [Hz]. After performing the symbol direction interpolation, the carrier direction interpolation is performed by using only the channel characteristics at the 3nth carriers. Hence, according to the sampling theorem, the carrier direction interpolation is performed using a filter whose passband is equal to or smaller than Tu/3 [s] as shown in FIG. 47B. For this reason, in the case of four symbol estimation, the carrier direction interpolation can be performed without being affected by aliasing as long as the delay spread is equal to or smaller than Tu/3 [s]. As set forth above, in the case of four symbol estimation, the channel characteristics can be estimated if the Doppler broadening is equal to or smaller than 1/(4 Ts) [Hz] and the delay spread is equal to or smaller than Tu/3 [s].
On the other hand, in the case of one symbol estimation, the interpolation is not performed in the symbol direction; hence, according to the sampling theorem, the first symbol estimation can be performed without being affected by aliasing as long as the Doppler broadening is equal to or smaller than 1/Ts [Hz]. Also, in the case of one symbol estimation, the carrier direction interpolation is performed by using only the channel characteristics at the SP carriers that are eleven carriers apart from one another in each symbol. Therefore, according to the sampling theorem, the carrier direction interpolation is performed using a filter whose passband is equal to or smaller than Tu/12 [s] as shown in FIG. 48. For this reason, in the case of one symbol estimation, the carrier direction interpolation can be performed without being affected by aliasing as long as the delay spread is equal to or smaller than Tu/12 [s]. As set forth above, in one symbol estimation, the channel characteristics can be estimated as long as the Doppler broadening is equal to or smaller than 1/Ts [Hz] and the delay spread is equal to or smaller than Tu/12 [s].
Non-Patent Literature 1 discloses technology for performing two-dimensional adaptive interpolation in the symbol and carrier directions. This technology involves adaptive interpolation using a Wiener filter, and allows (i) calculating optimum filter coefficients based on the maximum delay and the maximum Doppler frequency and (ii) performing the interpolation based on the calculated optimum filter coefficients by using values of channel characteristics at carriers to which the pilot signals are allocated, the channel characteristics being calculated based on the pilot signals.
More specifically, based on the maximum delay amount τmax and the maximum Doppler frequency fDmax, the receiver calculates an autocorrelation matrix and cross-covariance vectors that are shown in the following (Equation 3) to (Equation 6). Thereafter, based on the calculated autocorrelation matrix and cross-covariance vectors, the receiver calculates filter coefficients using the following (Equation 7). Then, the receiver performs the interpolation based on the calculated filter coefficients by using the values of channel characteristics at the carries to which the pilot signals are allocated, the channel characteristics being calculated based on the pilot signals.
                                              ⁢                                            θ                                                Δ                  ⁢                                                                          ⁢                  t                                ,                                  Δ                  ⁢                                                                          ⁢                  f                                                      ⁡                          (                                                k                  -                                      k                    ″                                                  ,                                  l                  -                                      l                    ″                                                              )                                =                                                    θ                                  Δ                  ⁢                                                                          ⁢                  t                                            ⁡                              (                                  k                  -                                      k                    ″                                                  )                                      ⁢                                          θ                                  Δ                  ⁢                                                                          ⁢                  f                                            ⁡                              (                                  l                  -                                      l                    ″                                                  )                                                                        (                  Equation          ⁢                                          ⁢          3                )                                                          ⁢                                            θ                              Δ                ⁢                                                                  ⁢                t                                      ⁡                          (                                                k                  ′                                -                                  k                  ″                                            )                                =                      si            ⁡                          (                              2                ⁢                π                ⁢                                                                  ⁢                                  f                  Dmax                                ⁢                                                      T                    s                                    ⁡                                      (                                          k                      -                                              k                        ″                                                              )                                                              )                                                          (                  Equation          ⁢                                          ⁢          4                )                                                          ⁢                                            θ                              Δ                ⁢                                                                  ⁢                f                                      ⁡                          (                                                l                  ′                                -                                  l                  ″                                            )                                =                      si            ⁡                          (                              2                ⁢                π                ⁢                                                                  ⁢                                  τ                  max                                ⁢                Δ                ⁢                                                                  ⁢                                  F                  ⁡                                      (                                          l                      -                                              l                        ″                                                              )                                                              )                                                          (                  Equation          ⁢                                          ⁢          5                )                                          Φ          ⁡                      (                                                            k                  ′                                -                                  k                  ″                                            ,                                                l                  ′                                -                                  l                  ″                                                      )                          =                                                            N                0                                            E                s                                      ⁢                          δ              ⁡                              (                                                                            k                      ′                                        -                                          k                      ″                                                        ,                                                            l                      ′                                        -                                          l                      ″                                                                      )                                              +                                                    θ                                  Δ                  ⁢                                                                          ⁢                  t                                            ⁡                              (                                                      k                    ′                                    -                                      k                    ″                                                  )                                      ⁢                                          θ                                  Δ                  ⁢                                                                          ⁢                  f                                            ⁡                              (                                                      l                    ′                                    -                                      l                    ″                                                  )                                                                        (                  Equation          ⁢                                          ⁢          6                )                                                          ⁢                                                            ω                _                            0              T                        ⁡                          (                              k                ,                l                            )                                =                                                                      θ                  _                                T                            ⁡                              (                                  k                  ,                  l                                )                                      ⁢                                          Φ                _                                            -                1                                                                        (                  Equation          ⁢                                          ⁢          7                )            
In the above manner, an interpolation filter can be formed according to the receiving environment that the receiver is in, and the passband of the interpolation filter can be changed to alleviate the delay spread and Doppler broadening. Therefore, when the extents of the delay spread and Doppler broadening are small, the passband of the interpolation filter can be adaptively narrowed. This results in removal of noise components and improvements in accuracy of channel characteristics estimation.